This invention relates to a driving force control apparatus for use with a motor vehicle and, more particularly, to such a driving force control apparatus adapted to reduce the driving forces applied to the driving wheels when slip occurs on the part of the driving wheels.
For example, Japanese Patent Kokai No. 63-12842 discloses a driving force control apparatus installed on a front-wheel-drive type automotive vehicle for providing a driving force reduction control (traction control) to reduce the driving forces applied to the front wheels (driving wheels) when slip occurs on the part of the front wheels. For such a driving force reduction control, conditions, such as revolution speeds, of the front and rear wheels are monitored while the vehicle is traveling for use in calculating a parameter representing the degree of driving slip of the front wheels. The parameter includes the slip ratio of the front wheels, the slip ratio changing rate of the front wheels, etc. When the calculated parameter exceeds a predetermined threshold value, the driving forces applied to the front wheels are reduced to suppress the driving slip of the front wheels.
However, the prior art driving force control apparatus performs the driving force reduction control not only when slip occurs only on the front wheels, but also when slip occurs on both the front and rear wheels. A reduction of the driving forces applied to the front wheels for slip on both the front and rear wheels will cause an increase of the gripping forces of the front wheels with respect to the load surface. This, in turn, increases the cornering force and the spinning moment directed inwardly of the circle in which the vehicle is turning so as to cause the vehicle to spin about the front wheels. This is true particularly when lateral slip of the rear wheel occurs.